With many of the services and applications provided in modern mobile radio systems, messages are intended to be transmitted not just to one, but to two and more mobile radio subscribers. Examples of such services and applications are newsgroups, videoconferences, video-on-demand, distributed applications, etc. During the transmission of messages to the different subscribers it is possible to send a copy of the data separately to each recipient. This technique, though simple to implement, is however not suitable for large groups. As the same message is transmitted over N (where N=number of recipients of the message) individual connections (unicast connections) and is also sent a number of times over common connection paths, this method requires a very high bandwidth.
In this case multicast transmission offers a better alternative. With this method, the various subscribers to whom the same message is to be transmitted are combined to form a group (multicast group) to which an address (multicast address) is assigned (point-to-multipoint transmission). The data to be transmitted is then sent only once to this multicast address. In the ideal case the multicast message is sent only once from the sender to the recipients over common connection paths. In this process the sender does not need to know the location of the recipients or how many recipients are covered by the multicast address.
No multicast transmission has been specified to date in UMTS (Universal Mobile Telecommunication System). In the UMTS mobile radio system the transmission of information to a user is effected by reservation of a physical resource. In the mobile radio service a distinction is made between two transmission directions for the transfer of data, irrespective of which type of data is involved. The data transmission from the typically stationary base station (designation in GSM-Global System for Mobile Communications) or NodeB (designation in UMTS) to the mobile terminals is generally referred to as transmission in the downlink direction DL, whereas the data transmission in the opposite direction, from a terminal to the base station, is known as transmission in the uplink direction UL. In UMTS two modes are provided for transmission over the air interface: In FDD mode the transmission in uplink and downlink directions takes place at different frequencies, whereas in TDD mode only one carrier frequency is used. Time slots are assigned to separate the uplink and downlink directions. In both modes the subscribers are separated by superposition of orthogonal codes (channelization codes) onto the information data. This multiple access method is known as the CDMA method. According to the current specifications (among other sources see TS 25.211 V4.0.0: Physical channels and mapping of transport channels onto physical channels, 3GPP-TSG-RAN, 2001-3 and TS 25.212 V4.0.0: Multiplexing and channel coding, 3GPP-TSG-RAN, 2001-3) of the UMTS-FDD mode, a physical channel, i.e. a radio channel, is defined in the downlink direction by carrier frequency, scrambling code, channelization code, and a start and stop time. The scrambling codes are used to scramble the already spread data. This is intended among other things to minimize the interferences in neighboring cells.
In UMTS there are two types of radio channels for transmitting information, called dedicated channels and common channels. With the dedicated channels a physical resource is reserved only for the transmission of information intended for a specific subscriber device only (user equipment). With the common channels information intended for all subscribers can be transmitted (e.g. the broadcast channel BCH) or for a specific subscriber only. In the latter case an indication of which subscriber the information is intended for must also be transmitted on the common channel.
Also part of the prior art is the SIR-based (Signal-to-Interference Ratio) closed-loop power control, which will be described briefly below (see also TS 25.214 V4.0.0: Physical layer procedures, 3GPP-TSG-RAN, 1 2001-3). The mobile radio device makes an estimate of the SIR for the dedicated channels which it receives. This value represents a quality criterion for the received signal. By comparison with a predefined value SIRtarget, a TPC (Transmission Power Control) command is generated and the result sent to the base station over an uplink channel. The value of SIRtarget is here predefined individually for each mobile station by the network such that adequate quality is guaranteed for the respective connection. The TPC commands are pure 1-bit items of information and constitute solely the information concerning whether the measured SIR is above or below the predefined value SIRtarget. For this reason the TPC commands can also be equated with the commands “UP” (SIR below SIRtarget) and “DOWN” (SIR above SIRtarget). The command “UP” means that the reception quality is inadequate and therefore an increase in transmission power is necessary at the transmitter. The converse applies to the “DOWN” command.
Group or multicast messages are usually sent via dedicated channels from the base stations or NodeBs disposed in the radio cells to the subscriber devices (mobile radio stations or mobile stations in GSM, user equipments UEs in UMTS; an appended “s” is used to form the plural) of the multicast group. In order to adjust the power of this message transmission or to transmit at sufficient and not too high power, an associated uplink channel is available per subscriber device, which channel informs the base station by means of TPC bits (transmission power control bits) that the transmission power is too high or too low for the respective subscriber device. The uplink-dedicated channel UL-DCH which transmits the TPC bits is usefully also controlled in terms of power so that the interferences in the mobile radio cell due to the UL transmission by a plurality of subscriber devices are kept as low as possible. As is known, this control is effected in turn by a dedicated DL channel associated with the UL channel. In UMTS an associated DL channel is assigned in this case to each subscriber device. This disadvantageously results in system resources being occupied with regard to spreading codes.